The present invention relates to a bistable optical device using two coupled lasers operating at different wavelengths. It is used in optics, more particularly in integrated optics.
A bistable system is a system having two stable states, the transfer from one state to the other taking place by the temporary modification of certain conditions applied to the system. When these conditions remain unchanged, the system remains indefinitely in one or other of these two states.
Bistable systems are widely used in electronics in the storage of binary signals, the formation of pulses with a steep front or the switching of signals.
Recent developments in optics and particularly integrated optics has led to a need for devices of this type, but which are optical and not electronic.
Hybrid optoelectronic systems are already known and use an intermediate process of an electronic nature between two optical processes. For example optoelectronic transfer is brought about by means of a photodetector and the electronooptical return by means of a light modulation system. The bistability function is devolved to the intermediate electronic process. Naturally such systems are complex, as well as being slow.
In addition, purely optical devices are known, which have the property of bistability. These are systems constituted by a Fabry-Perot resonator in which is inserted a material, whereof one of the optical properties (absorption, refractive index, etc.) can be modified by auxiliary light radiation. The materials frequently used for this purpose have a saturable absorption, i.e. an absorption which decreases in the case of an increase of the light intensity of the radiation passing through them. Such a bistable device is described, for example, in U.S. Pat. No. 3,610,731 of H. Seidel, entitled "Bistable optical circuits using saturable absorber within a resonant cavity".
Such systems have a transmission by stability in that it is the transmitted light intensity which has two different values, one being high when the absorbent is not saturated and the other high when the absorbent is saturated by the auxiliary beam.
However, a device of this type has two major disadvantages:
the light intensity of the output beam is, for these two states, below the intensity of the input beam, whilst the intensities of the available beams are not the same for both states, which is an obstacle to the series-arrangement of several bistable devices; PA0 for at least one of the two stable states (that which in the circumstances corresponds to a high transmitted intensity) it is necessary for the device to be permanently supplied by an auxiliary light beam, which requires an external auxiliary source, e.g. a laser. PA0 either on the resonators of the lasers to modify their losses (reduction of the losses of the laser which does not emit or increase of the losses of the emitting laser), for which purpose electrooptical or acoustooptical modulators can be used, like those used in triggered lasers; PA0 or on the amplifying media to modify their gain (reduction of the gain of the emitting laser or increase of the gain of the laser which does not emit), which can easily be obtained by modifying the injection current intensity, it also being possible to reduce the gain of one of the amplifying media by means of an auxiliary light beam passed through it in accordance with the very principle of the device, whereby this auxiliary beam can come from another bistable optical device of the same type, so that the bistable devices can be connected in cascade leading to an "all-optical" chain of bistable devices.
Other bistable optical devices are known, which have been designed to obviate these disadvantages. These devices use two identical lasers, one emitting through the amplifying medium of the other and vice versa. Thus, the two lasers are in competition and only one of them can oscillate to the detriment of the other. The following mechanism leads to bistability.
Each appropriately excited amplifying medium is the seat of a population inversion, which gives the said medium the capacity to amplify radiation. For a laser to oscillate, it is necessary that the gain of its amplifying medium preponderates over the losses of the resonator. When one of the lasers is in this situation, it emits a light beam, which passes through the amplifying medium of the other laser. This beam is amplified by the second laser, which has the effect of reducing the population inversion inherent in the second laser. Thus, the gain of the amplifying medium of the second laser is reduced. This gain can drop to a value inadequate for compensating the losses of the resonator, so that the second laser is inhibited by the first. Thus, the system is in a first state in which only the first laser oscillates.
If the first laser stops emitting for a short time, either because its gain is artificially lowered to a value below the losses, or because the losses are increased, the gain saturation phenomenon of the second laser is ended and this laser is under favourable conditions to oscillate. It is then the second laser which emits a light beam. This beam traverses the amplifying medium of the first laser, whose gain drops below the threshold, which prevents it from oscillating. The system is then in a second state corresponding to the oscillation of the second laser.
Thus, such a device can be in one or the other of two states, depending on whether one of the lasers is emitting or not. Thus, such a device has an emission bistability and not an absorption bistability. Moreover, it has a perfect symmetry, because the two light beams corresponding to the two states of the device have the same intensity, which obviates the first disadvantage referred to hereinbefore. Moreover, the switching between states is obtained by a very brief action on one of the lasers and does not require a permanent auxiliary source. Therefore it also obviates the second disadvantage referred to hereinbefore.
A bistable device of this type is described in U.S. Pat. No. 3,760,201, granted on Sept. 18, 1973 and entitled "Optical flip-flop element", as well as in the article entitled "Mutually quenched injection lasers as bistable devices" published by G. J. Lasher and A. B. Fowler in IBM Journal of Research and Development, vol. 8. no. 9, September 1964, New York.
However, despite the interest of such devices a serious disadvantage remains. Thus, with such devices the two light beams emitted by the device differ from one another by their direction, which are generally orthogonal. Thus, the beam emitted by one of the lasers passes through the amplifying medium of the other in a transverse manner. This clearly presupposes that the width of the light beam emitted by one of the lasers is of the same order of magnitude as the length of the amplifying medium to ensure that the passage through said medium by the beam leads to significant effects. However, as the device is symmetrical, this implies that the width of the amplifying medium must be roughly equal to its length. In other words, such a device must use amplifying mediums with a substantially square cross-section. This is described in the two documents referred to hereinbefore.
However, such a structure is totally unsuitable for semiconductor lasers in which the amplifying medium is in the form of a strip, whose width is much smaller than its length. The width-length ratio can even drop to values of about 1:100 in the case of lasers used in integrated devices for optical telecommunications. Consequently there is no question of transmitting light beams transversely into the amplifying media, because only minimal effects would result.